Accumulation of desirable alleles in a population through various breeding techniques is known as population improvement and those breeding procedures that are used for such work are referred to as population improvement approaches.
In plant breeding, following four methods are used for population improvement:
1. Recurrent selection
2. Disruptive selection
3. Diallel selective mating, and
4. Biparental mating.
Method # 1. Recurrent Selection:
Recurrent selection refers to reselection generation after generation with intermating of selects to provide for genetic recombination. It is an important method of improving the frequency of desirable alleles in a plant breeding population. This method was originally developed for genetic improvement of cross pollinated species. Now it is also used for population improvement in self- pollinated species.
Method # 2. Disruptive Selection:
This is another breeding technique which is used for development of superior genotypes in a population. In this breeding approach, both extreme types for a character are selected in the segregating population and intermating is done among selected plants. Suppose we want to develop early maturing genotypes.
To achieve this goal, plants with extreme earliness and lateness are selected in F2 or later generations of a cross and mating is done between early and late plants. Several selections and mating cycles are made to achieve the desired improvement. The concept of disruptive mating and selection was developed by Mather (1953) and Thoday (1958, 1960).
The main features of disruptive mating and selection are given below:
i. Application:
This method is applicable to the genetic improvement of both self and cross pollinated species.
ii. Disruptive selection may be considered a form of recurrent selection because this method also involves selection and intermating of superior plants in segregating population.
iii. It differs from the directional selection. In case of directional selection, plants are selected only from one extreme and are intermated for further selection. Disruptive selection does not change mean performance of a population, whereas mean performance is improved by directional selection.
Breeding Procedure:
A cross between two different varieties is made. The F1 is raised and seed is harvested to grow F2. In F2, plants with two extremes of a character say extremely early and extremely late for maturity are selected. Crosses are made between early and late plants. This completes one cycle of selection and mating.
The crossed seed is grown and the second cycle of selection is completed. Many cycles of selection and mating are made to achieve the desired improvement in the character under consideration. At the end, progeny of selected plants are grown and the best progeny is selected.
Merits and Demerits:
Merits:
i. Disruptive selection is an efficient breeding method for breaking undesirable linkages, releasing genetic variability, generating diversity and improving the adaptation of plant populations.
ii. Disruptive selection is useful when wild germplasm is utilized in the breeding programmes. It will break repulsion phase linkages in such breeding programmes.
There are two main demerits of this method. Firstly, repeated crosses have to be made for improvement of a particular character. Secondly, populations of limited crosses only can be handled by this method at a time.
Achievements:
This technique has been used for the development of superior genotypes in crops like Sorghum, brown mustard and cotton. In cotton, this technique has helped in development of early material and improvement of seed oil content.
Method # 3. Diallel Selective Mating System (DSM):
The concept of diallel selective mating system (DSM) was originally developed by Jensen in 1970. Jensen used this system for genetic improvement of wheat. Now this system is used for genetic improvement of various autogamous crops especially small grain crops such as wheat, barley, rice, etc.
The main features of this system are briefly presented as follows:
i. Application:
This is an effective method of improving those autogamous species which are difficult to cross and have few seeds per cross. Thus, it is useful in improving small grain crops. This is an important method of population improvement in self-pollinated species. Thus, it supplements conventional breeding methods in autogamous crops.
ii. Approach:
This is generally considered as a form of recurrent selection, because it involves selection and intermating in segregating generations. Both mass and recurrent selection procedures are used for handling of material in this system. Thus, it permits use of conventional breeding method as well as improvement of population by intermating.
iii. Advantage:
In this system, superior or promising genotypes can be identified and isolated for development of new cultivars at any stage of breeding programme. In other words, this system permits extraction of new cultivars at various stages. This system also permits incorporation of new germplasm at any stage in the breeding programme.
iv. Parents Involved:
In the conventional breeding methods, usually two parents are included in the breeding programme at a time. This system permits inclusion of multiple parents in the breeding programme.
v. Goals:
This system fulfills both short-term and long-term breeding goals, because it permits use of conventional selection procedures at any stage in the breeding programme as well as broadens the genetic base of populations.
vi. End Product:
The end product of DSM can be used as a pure-line variety, a mass selected variety or as parental lines for further use in the breeding programme.
Breeding Procedure:
The breeding procedure of DSM consists of three major steps, viz.:
(a) Parental diallel series,
(b) F1 diallel series, and
(c) Selective mating series (Fig. 24.1).
These are briefly discussed below:
(a) Parental Diallel Series:
The parental lines are selected keeping in view the breeding objectives. The selected parents are crossed in a diallel fashion and F1 seeds are obtained. If the number of parents is 7 or less, complete diallel is used. When the number of parents is more than 7, partial diallel cross is made. The parental diallel series is designated as P1.
(b) F1 Diallel Series:
The above F1 crosses are used in two ways, viz., (i) to produce F2 population, and (i) to develop F1 diallel series. Mass selection is practised to advance the F2 population. The selected F1 crosses are crossed in a diallel fashion (upto 7 complete diallel and above 7 partial diallel). The F1 diallel series is also known as multiple parents crosses because each cross involves four parents. The F1 diallel series is designated as P2.
(c) Selective Mating Series:
The F1s from the multiple crosses are selfed to produce F2 population. The F2 population is used in two ways, viz. (i) To produce F3 population by mass selection, and (ii) To develop first selective mating series (P3) by intermating selected F2 plants.
The F1 crosses developed by selective mating are used in three ways. Some are selected for intermating to form the second selective mating series (P3) and others are crossed with new parents (germplasm lines) not included in the original diallel.
This helps in broadening the genetic base of breeding populations. Some plants are selfed to produce F2 population. Many such cycles can be made to achieve the desired breeding objective.
Merits and Demerits:
Merits:
i. DSM is very much useful in broadening the genetic base of populations by incorporation of multiple parents (germplasm lines) in the breeding programme.
ii. This system is very effective in breaking undesirable linkage blocks, because it permits intermating of selected plants in segregating generations. Intermating in segregating populations provides greater opportunities for additional gene recombination’s. In other words, DSM fasters the genetic recombination.
iii. This system also results in creation of vast genetic variability for various economic characters. This is possible due to incorporation of multiple parents in the breeding programme and intermating in segregation populations.
iv. In small grain crops, this system has been reported to be very effective in developing new cultivars.
Demerits:
i. In this system, large numbers of crosses have to be made to obtain sufficient quantity of crossed seeds. This is a difficult task without the use of male sterility.
ii. This system involves handling of material by mass selection as well as intermating in segregating populations which requires more labour and space.
iii. This method is less effective in improving those characters which have low heritability.
iv. This system is rarely used in crop improvement and has not become very popular so far.
Method # 4. Biparental Mating:
Biparental mating refers to crossing among randomly selected plants in F2 or subsequent generation of a cross in a definite fashion. The concept of biparental mating was originally developed by Comstock and Robinson (1948, 1952). It is an important method of concentrating favourable genes in a population. Three or four cycles of biparental mating are required to achieve the desired improvement.
Merits and demerits of this technique are given as follows:
Merits and Demerits:
Merits:
i. It is very effective in breaking undesirable linkages. Mating of randomly selected plants in segregating population provides greater opportunities of recombination between linked genes.
ii. This technique leads to creation of vast genetic variability in a population for various economic characters.
iii. This technique is applicable for genetic improvement of both self and cross pollinated species.
iv. This is an easy and effective method of population improvement.
Demerits:
i. Large segregating population (F2 or F3) has to be grown for selection and intermating to achieve desired improvement.
ii. Repeated biparental mating has to be made for 3-4 generations.
iii. This technique permits handling of limited segregating populations at a time.
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